The present invention relates to a conductivity modulated MOSFET.
A conductivity modulated MOSFET has a drain region whose conductivity type is opposite to that of a source region of a normal power MOSFET. Typical examples of conventional conductivity modulated MOSFETs are described by M. F. Chang et al., "25 AMP, 500 Volt Insulated Gate Transistors", 1983 IEEE IEDM Technical Digest pp. 83-86 and by A. M. Goodman et al., "Improved COMFETS with Fast Swtiching Speed and High-current Capability", 1983 IEEE IEDM Technical Digest pp. 79-82.
Such a conductivity modulated MOSFET has a parasitic p-n-p-n thyristor. When the parasitic thyristor is turned on, the MOSFET cannot be turned off even if a voltage across the gate and source thereof is zero. The element is often damaged. The parasitic thyristor is turned on since holes injected from the p.sup.+ -type drain layer into an n.sup.- -type drain region pass into the source electrode through a p-type base layer. In other words, when a hole current flows through the p-type base layer and a voltage drop across a resistor of the base layer immediately under the source layer exceeds 0.7 V, electrons are injected from the source layer to turn on the parasitic thyristor. This phenomenon is described in the above-mentioned "25 AMP, 500 Volt Insulated Gate Transistors".
In order to prevent such a latch-up phenomenon, various proposals have been made including the two papers described above. The present invention, however, has been made to prevent the latch-up phenomenon from another viewpoint.